162 C. Barus— Measurement of Fog Particles. 



tubes led to the " curl" aneroid, tlie filter, and to the cock for 

 influx of air, the only large tube being the exhaust pipe. 

 Condensation again occurred as a microscopically granular 

 deposit spontaneously on the raised surface, under all circum- 

 stances, and the experiments were failures. After oiling the 

 raised filmy mica surface, however, droplets were often seen 

 to fall and either to stick fast or to float. These could at 

 times be counted (2000-5000 per cub. cm.) ; but the rapid 

 evanescence of precipitated droplets and the failure of all 

 attempts to reach systematic results induced me to abandon 

 the capsule. 



I therefore returned to the apparatus in figure 1, using at s a 

 plate of thin microscopic glass covered with a film of oil and 

 exposed in the capacious vacuum chamber. The experiments 

 were now phenomenally successful. Thus for the aperture s — 5 

 the mean results were n — 150,000, and for s = 4'6 (wgb p), 

 n = 140,000. The precipitation of globules was clearly seen 

 and they persisted even after the exhaustion was removed. 

 The numbers being excessive and referable to globules swept 

 in by lateral air currents, an improvement was now added by 

 increasing the diameter of the disc p to about 5 cm . The 

 improvetl apparatus gave no results whatever, and the mere 

 addition of the wider disc wiped out all precipitation. But 

 this capricious behavior is characteristic, for next day with a 

 smaller disc drops were seen to fall as follows : 



s = 4-5 wgbp w = 6-5Xl0 4 



4*6 wgbp 4:'1 



6*3 wgbp 13-3 



after which no precipitation could be caught in the six subse- 

 quent exhaustions by the identical method. The same unac- 

 countable irregularity was noted in the afternoon. Next day 

 again the first experiments showed 



s = 6'0 wcg n = IS X 10* 



6*4 wgvp 12*4 



after which further precipitation did not occur. 



The apparatus was then again modified to the final form 

 shown in figure 1, by inserting a thin brass tube laterally 

 through the stopper C, and firmly soldering this tube at e 

 above and below to the body of the microscope. A rod 

 snugly fitting the tube thus provided an eccentric focussing 

 device, abed, with a stuffing box at b, and an external handle 

 at a. The latter is adjustable by aid of a set screw so that the 

 plate s may be kept in focus during rotation of the rod. To 

 catch the droplets, the plate s is rotated into the position s r 

 quite beyond the shield, p, for a time (15 — 30 seconds) and 

 then returned to s for examination. In this way the definite 



